It is known that beating the modes of two single frequency lasers of different wavelength on a fast photodiode will result in the generation of an RF beat frequency. The frequency of the beat signal is defined by the separation in wavelength of the light generated by the two lasers. Such an approach has been used for the generation of RF carriers for optical communications. In much of the prior art this is accomplished by the use of two or more discrete devices as disclosed in U.S. Pat. No. 5,710,651 (Logan), U.S. Pat. No. 5,379,309 (Logan) and U.S. Pat. No. 5,838,473 (Wake). This approach, while feasible, requires the accurate control of two discrete devices, which increases the complexity and therefore the cost of the required control. In Electronics Letters, 1995, Vol. 31 No. 5 pp 364-365 (C. R. Lima, D. Wake and P. A. Davies), the use of a dual frequency distributed feedback (DFB) laser is proposed. In this approach both wavelengths are generated within one device by the use of lasing on either side of the stop band of a DFB. This approach limits the frequency difference to around 60 GHz due to the limitations of the strength of grating that may be employed.
It is also recognised that the beating of two wavelengths from different or the same semiconductor laser places limitations on the phase noise of the beat frequency. U.S. Pat. No. 5,379,309 (Logan) proposes overcoming this by the use of an additional mode locked laser. While Lima, Wake and Davies achieve this by providing an electrical drive signal at a sub-harmonic of the beat frequency.